1. Field of the Invention
The present invention relates generally to multiple band radio receivers, and relates more specifically to a front end for a digital radio that can tune in digital and analog signals.
2. Description of Related Art
New digital radio receivers supporting In Band On Channel (IBOC) or High Definition (HD) Radio, are capable of detecting, tracking, and processing a digital broadcast signal in addition to the traditional analog signal for any given channel. FIG. 7 illustrates a hybrid IBOC signal with different bands related to digital and analog radio signals. The spectral location of IBOC digital subcarriers drives a new requirement for wide intermediate frequency (IF) filters, allowing the wider band signal to be properly passed for detection and decoding. The different requirement for IF filters present new challenges to the radio front end design since the analog signal is typically filtered in the tuner using a narrow IF filter, while a wide IF filter is used to pass a digital signal located on the same AM/FM channel.
To receive both analog and digital signals in a hybrid IBOC signal, today's radio front end designs have changed in a couple key ways. First the tuner has changed in terms of IF signal filtering. FIG. 1 shows a typical AM/FM tuner architecture. Filter boxes 12, 13 represent tuner areas that have been affected by new HD Radio requirements. FIG. 2 shows IF filter configurations 22-26 that can be used to provide narrow IF filtering for traditional AM/FM and wide filtering for IBOC. FIG. 2 illustrates general system designs representative of a number of the types of configurations that can be used. The combination of filters and control switches typically adds complexity and cost to the tuner.
The second major change to radio front end designs is related to the IF signal path. previously a single IF signal path was adequate for AM/FM signal processing. With the increased operability posed by HD Radio for signal detection and decoding, an additional path is used to accommodate a wide IF signal path in addition to the conventional narrow IF signal path. The implementation of multiple IF paths can be accomplished in several ways, including multiplexing signal paths so that one IF signal is output from the tuner, or providing separate wide and narrow IF signals. FIG. 3 shows examples of front end configurations 32, 33 found on HD Radio receivers today.
Present methods attempt to provide a solution at the tuner, through the use of multiple tuners or multiple IF outputs, or through a combination of tuner and host microcontroller interaction where the microcontroller tries to determine when to switch between wide/narrow filters during all modes of operation.
The most typical front end strategies for IBOC receivers today are;
(1) Two tuners, one with narrow IF filters & one with wide IF filters; separate wide IF and narrow IF output paths, and two IF analog/digital converter (ADC) stages for each IF path.
(2) Single tuner with separate wide & narrow IF filters; separate wide IF and narrow IF outputs; two IF ADC stages for each IF path.
(3) single tuner with selectable wide & narrow filters, selected via a programmable switch. The tuner output is a single IF analog signal to an IF ADC. Control is difficult to implement in this strategy, particularly tor FM signals, due to lack of digital information in a narrow filtered (narrow IF) signal. The lack of information poses a problem for the system designer and software engineer, who must use “guessing” methods to determine if a digital signal is available and whether a switch should occur between narrow & wide filters in the tuner. These methods are not always reliable, and the extra control software added to the host microcontroller is undesirable.
Each of the above strategies, illustrated in FIG. 2 as configurations 22-26, has drawbacks, either in terms of cost or complexity in implementation. For example, configurations 24-26 use multiple ADCs, while configurations 22-23 represent additional complexity in switching between wide and narrow filter outputs. In each configuration, the filters operate on the analog IF signal, and provide one or more ADCs on the outputs of the filters. It would be desirable to attain a simplified strategy that minimizes the IF signal path, both in terms of tuner filter stages and number of IF signal outputs from the tuner.
In particular, it would be desirable to obtain a simplified system that is capable of detecting, tracking, and processing a digital broadcast signal in addition to the traditional analog signal for any given channel.